docs: new design for the documentation site (#5934)

* docs: new design for the documentation site

Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono
served by the site (upright and real italic), brand colours, blue header band
with the syntax pattern, wordmark in the header, new home page (hero, cards,
animated feature tour), 404 page, polish for tabs, search, footer, focus
rings, tables and code blocks, and theme features (code copy, previous/next
footer, TOC follow, search suggestions).

Draw the number railroad diagram, the parser callback events and the iterator
illustrations as inline SVGs that follow the light and dark scheme. The
generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow
(json-light.svg, json-dark.svg, replacing json.gif on the home page) is built
by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become
hexagons, which need far less room.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: add figures, table captions and a macro chooser

- Figures with numbered captions: SAX stepper, json_view layout and edit
  stepper, exception map, JSON pointer and patch walkers, QA pipeline,
  scaling and binary size charts, roadmap lanes, release timeline,
  integration decision tree and namespace builder
- Table captions (nj-table-caption) on 9 pages
- Compile times: modules and PCH charts, filesystem section, re-measured
  Figure 1, links between features/modules.md and compile_times.md
- Interactive macro chooser for features/arbitrary_types.md
- Link check: exclude URLs that answer the plugin with 400/401/419 or are
  dead (customers.md)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: macro chooser generates adl_serializer for non-default-constructible types

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: address review comments on the new design

- Home: add a "Latest release" button
- Sponsors: update the list and group it by tier
- Features: list all feature pages on the index page
- Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline
- Exceptions: prefix all example messages with the exception name
- API: no code font for format names in the See also links of the to_/from_ pages
- Merge Patch: show the document before, the patch, and the result
- Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/load.md

@@ -16,14 +16,14 @@ static basic_json_document load(std::vector<std::uint8_t>&& image,
 
 1. Reads an image [`save()`](save.md) wrote, from a pointer and a byte count. The image is **borrowed**: `image`
    must stay alive and unchanged for as long as the returned document, and any view taken from it, is used.
-2. Reads an image from a `#!cpp std::vector`. Also **borrowed** -- equivalent to overload 1 called with
+2. Reads an image from a `#!cpp std::vector`. Also **borrowed** — equivalent to overload 1 called with
    `#!cpp image.data()` and `#!cpp image.size()`.
 3. Reads an image, keeping the vector instead of copying it: `image` is moved into the document (no copy), which
    then owns it for as long as it needs the text and the decoded strings. [`owns_source()`](owns_source.md) is
    `#!cpp true` afterward.
 
-In every overload, the node index is copied into storage the document itself owns -- so that it is properly aligned,
-and, for an [editable](index.md#edits) document, can be edited -- while the text and the decoded strings stay in
+In every overload, the node index is copied into storage the document itself owns — so that it is properly aligned,
+and, for an [editable](index.md#edits) document, can be edited — while the text and the decoded strings stay in
 `image`. The hash indexes [large objects](../../features/json_view.md) use for lookup are rebuilt, exactly as after
 parsing.
 
@@ -49,7 +49,7 @@ Overloads 1 and 2 give the strong guarantee: `image` is only read, never written
 caller's buffer untouched.
 
 Overload 3 moves `image` into the document *before* validating it, so that a good image is kept without a copy. If
-loading then fails, the partially built document -- and the vector now inside it -- is discarded along with the
+loading then fails, the partially built document — and the vector now inside it — is discarded along with the
 exception, and `image` itself is left **empty**, not restored to what was passed in. Move a copy in instead, or
 validate with overload 2 first, if the original vector must survive a failed load.
 
@@ -63,7 +63,7 @@ Throws [`parse_error.116`](../../home/exceptions.md#jsonexceptionparse_error116)
 | `too short`            | `image` is `#!cpp nullptr`, or `size` is smaller than the 64-byte header                             |
 | `unknown format`       | the header's magic bytes or version do not match, or a reserved header field is not zero             |
 | `sizes out of range`   | the node count, text size, or decoded-string size the header describes does not fit `size`, or the `#!cpp '\0'` after the text or after the decoded strings is missing |
-| `the check failed`     | `check` is not `#!cpp image_check::none`, and the image fails it -- see [`image_check`](#image_check) |
+| `the check failed`     | `check` is not `#!cpp image_check::none`, and the image fails it — see [`image_check`](#image_check) |
 
 !!! failure "Example messages"
 
@@ -116,7 +116,7 @@ How thoroughly `load()` validates `image` before trusting it.
     token is well-formed and matches the value stored for it. Strings and float tokens may share a range only if the
     ranges are identical (as nodes that share a value do), and never overlap otherwise. Reading and serializing a
     checked image is safe and always produces valid JSON, exactly as for a parsed document. This is the right choice for
-    an image from anything you do not fully control -- a file, a cache shared with other processes, a peer on the
+    an image from anything you do not fully control — a file, a cache shared with other processes, a peer on the
     network.
 
 `bounds`
@@ -136,7 +136,7 @@ How thoroughly `load()` validates `image` before trusting it.
     your own memory.
 
 **Lifetime.** Overloads 1 and 2 borrow `image`: it must stay alive and byte-for-byte unchanged for as long as the
-returned document, and any [view](../basic_json_view/index.md) taken from it, is used -- exactly like a document
+returned document, and any [view](../basic_json_view/index.md) taken from it, is used — exactly like a document
 [`parse()`](parse.md) borrowed its input for. Overload 3 avoids this by keeping the vector itself; see
 [`owns_source`](owns_source.md).
 
@@ -145,7 +145,7 @@ returned document, and any [view](../basic_json_view/index.md) taken from it, is
     The image format is versioned but not yet stable, and may change in an incompatible way before it is declared
     stable; `load()` already rejects an image written by a different format version with `parse_error.116`
     ("unknown format"). Use images to cache a document within one build of the library, or to hand one to another
-    process running the *same* build on the *same* (little-endian) machine -- not as a long-term storage format.
+    process running the *same* build on the *same* (little-endian) machine — not as a long-term storage format.
 
 ## Examples
 
@@ -154,8 +154,8 @@ returned document, and any [view](../basic_json_view/index.md) taken from it, is
     The example below saves a parsed document as an image, checks that `load()` reproduces the original
     [`dump()`](../basic_json_view/dump.md) without parsing, and shows the difference between
     `load(std::move(image))` (owned) and `load(image)` (borrowed). It then damages one byte of the image and shows
-    `image_check::full` rejecting it with `parse_error.116`, while `image_check::bounds` -- meant for a cache the
-    process already trusts -- still reads it without going out of bounds.
+    `image_check::full` rejecting it with `parse_error.116`, while `image_check::bounds` — meant for a cache the
+    process already trusts — still reads it without going out of bounds.
 
     ```cpp
     --8<-- "examples/basic_json_document__load.cpp"
@@ -169,9 +169,9 @@ returned document, and any [view](../basic_json_view/index.md) taken from it, is
 
 ## See also
 
-- [save](save.md) - write the document as an image
-- [owns_source](owns_source.md) - return whether the document holds its own copy of the text
-- [parse](parse.md) - deserialize from JSON text instead of an image
+- [`save`](save.md) - write the document as an image
+- [`owns_source`](owns_source.md) - return whether the document holds its own copy of the text
+- [`parse`](parse.md) - deserialize from JSON text instead of an image
 - [Images](../../features/json_view.md#images) - why and when to use images
 
 ## Version history